1. Field:
The invention is in the field of chemical processing of a spent acid solution used for pickling a metal, in particular zirconium or hafnium.
2. Description of the Prior Art:
Zirconium and hafnium metals and alloys are normally conditioned, following production and before shipment to users, by a pickling procedure in a nitric acid bath containing a relatively small percentage of hydrofluoric acid. The spent pickle acid, saturated with zirconium or hafnium fluoride, is customarily sent to waste after being neutralized by the addition of lime.
Proposals have been made heretofore for alleged commercially useful regeneration of the spent pickle liquor for reuse in the pickling circuit and, in some instances, for the recovery of useful by-products.
Thus, in Megy et al., U.S. Pat. No. 4,105,469, the spent pickle liquor is regenerated by adding sodium fluoride (NaF), which, in the case of zirconium, precipitates sodium zirconium fluoride (Na.sub.2 ZrF.sub.6) out of the solution. After hydrogen fluoride (HF) and nitric acid (HNO.sub.3) are added to the residual solution to make up losses thereof, the regenerated solution is recycled for reuse in the pickling circuit. The precipitant by-product can be used in the making of zirconium-magnesium alloys or can be reduced to zirconium metal.
To like effect is Fennemann et al., U.S. Pat. No. 4,330,342, which teaches precipitation of Na.sub.2 ZrF.sub.6 from a spent HF HNO.sub.3 pickle liquor by the addition of dissolved sodium hydroxide (NaOH) to such liquor after heating thereof, precipitation of the Na.sub.2 ZrF.sub.6 taking place after cooling of the so-treated liquor.
Pansom, U.S. Pat. No. 4,738,747, teaches how such a spent pickle liquor resulting from the etching of zirconium metal or an alloy thereof can be regenerated for reuse in the etching circuit by the addition of appropriate amounts of hydrofluoric acid and a nitric acid following measurements and calculations indicative of the correct amounts, this being accomplished without the previous removal of dissolved zirconium from the spent solution.